The present invention relates to a device for estimating the frequency offset, in particular for a mobile-telephone demodulator.
Frequency modulation is the modulation technique most widely used in mobile radio systems.
In frequency modulation, the signals have all their information contained in the phase or the frequency of the carrier.
In order to recover the data, the demodulator which is used must estimate the phase or the frequency of the signal. If the carrier frequency is not exactly the expected frequency (because of a difference in frequency between the frequency of the oscillators of the transmitter and of the receiver), this may lead to an inaccuracy, and therefore increase the error rate of the demodulator.
It will be assumed below that the oscillator of the receiver is perfect but the oscillator of the transmitter is imperfect. In reality, both are imperfect.
One of the great problems with radio systems is that of reducing the impact of the frequency error of the received signal by compensating for it locally.
Indeed, in order to reduce the total system cost, manufacturers are reluctant to use oscillators which are accurate and therefore expensive.
There are usually two ways of compensating for the frequency error of the carrier:
a) the frequency of the oscillator of the receiver is changed in order to adapt it to the frequency of the carrier.
This involves external compensation using a VTCXO;
b) the frequency is corrected directly inside the demodulator. This therefore involves internal compensation.
In both cases, it is necessary to have a good estimate of the frequency error in a probably imperfect environment (low signal/noise ratio) in a fairly short time.
Furthermore, if the receiver successively receives signals from different transmitters, it must frequently estimate new frequency errors.
In this case, the frequency error estimator must give a reliable estimate on the basis of a very small number of samples.
An aspect of the invention is to create a very efficient frequency error estimator used with a differential phase demodulator.
A further aspect of the invention is to create a frequency error estimator which is particularly well-suited to a xcfx80/4 QPSK demodulator, having four-state phase modulation with a jump of xcfx80/4, dedicated to wireless application such as PHS and PDC mobile telephones.
The invention therefore relates to a method for estimating the frequency offset in a signal received by a demodulator of a mobile-telephone set, characterized in that it includes the steps consisting in:
calculating an error signal xcex5 on the basis of the phase xcfx86 of the output signal of the demodulator and of the ideal phase xcfx86o of this signal;
calculating estimates of the averages of the signal xcex5(FO-RAW) and of its absolute value (Q-RAW) over a certain number of samples, taking a certain number of bits (nFO-RAW, nQ-RAW) of these estimates and applying them to means for modeling a function xcexa9(FO-RAW, Q-RAW) representing the frequency offset.